The present invention relates generally to an electronic ballast for powering a discharge lamp and a lighting fixture. More particularly, the present invention relates to an electronic ballast with adjustable filament preheating based on output current symmetry.
An electronic ballast as known in the art for lighting a hot-cathode type discharge lamp, such as a high-pressure discharge lamp also referred to as an HID (High-intensity discharge lamp), typically includes a power converting circuit for receiving a DC input and outputting AC power, and a control circuit for controlling the power converting circuit.
In one example of an electronic ballast as previously known in the art, the control circuit performs a filament heating operation by making an output frequency of the power converting circuit relatively higher to heat each filament of the discharge lamp upon lamp startup This heating operation takes place after a starting operation, during which an output voltage of the power converting circuit is relatively higher, and before the start of steady-state lamp operation.
The electronic ballast of this example can stabilize lamp discharge after a shift to steady-state operation and suppress lamp fade-out as compared to the case where filament heating is not performed.
Referring to FIG. 19(a), a starting period P1 is shown during which a lamp starting operation is performed. When a subsequent filament heating period P2 during which a filament heating operation is performed is relatively short as shown, a filament of the discharge lamp is not sufficiently heated before the start of a steady-state operation period P3 during which a steady-state operation is performed. This results in a current being output to the discharge lamp I_lamp (hereinafter referred to as “lamp current”) becoming asymmetrical between polarities. When the filament heating operation shifts to the steady-state operation where the filament of the discharge lamp is not sufficiently heated, as described above, lamp discharge becomes unstable after a shift to the steady-state operation, thereby possibly causing fade-out. Accordingly, it is necessary to set the filament heating period P2 to be sufficiently long as shown in FIG. 19(b). The necessary length of the filament heating period P2 (duration of the filament heating operation) varies for each discharge lamp.
However, where multiple discharge lamps may be connected to the ballast, a particular duration of the filament heating operation that accommodates all discharge lamps is likely to be excessive for most of these discharge lamps. Because the filament heating operation allows the power converting circuit to output larger power than the steady-state operation, in order to suppress a negative effect on the life of the discharge lamp the duration of the filament heating operation needs to be reduced as much as possible while yet properly heating the filaments.
A small lamp current also requires extension of the filament heating period P2 (i.e. a duration time of the filament heating operation).